There are a couple of cruises from San Francisco that are just 3 days long. Quick 3-day cruises are popular because they are easy to fit into a schedule and they are affordable.
A 3-day cruise is a good way to try out cruising if you've never been before. You'll get to experience all of the entertainment and food without committing too much time or money.
The thing that prevents 3-day cruises from San Francisco is the federal law that mandates that ships must stop in a foreign port. There aren't any foreign ports close enough to complete that quick round-trip.
For other options, see 3-day cruises out of Los Angeles. That is the main departure port in California for really short cruises.